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ABSTRACT Climate change can influence host–parasite dynamics by altering the abundance and distribution of hosts and their parasites as well as the physiology of both parasite and host. While the physiological effects of hosting parasites have been extensively studied in aquatic and laboratory model systems, these dynamics have been much less studied in wild terrestrial vertebrates, such as ectotherms that live in tropical forests. These organisms are particularly vulnerable to climate change because they have limited scope for behavioral buffering of stressful temperatures while already living at body temperatures close to their heat tolerance limits. Thus, it is imperative to understand how parasitism and tolerance to stressful thermal conditions, both of which are changing under climate warming, might interact to shape survival of non-model organisms. We measured heat tolerance and assessed endoparasites and ectoparasites in slender anole lizards (Anolis apletophallus; a lowland tropical forest species from central Panama). We then treated lizards with the antiparasitic drugs ivermectin and praziquantel and measured changes in immune function and heat tolerance compared with an unmanipulated control group. Immune function was not altered by treatment; however, heat tolerance increased in treated lizards. Additionally, higher endoparasite and ectoparasite abundance was associated with lower heat tolerance in a separate set of wild-caught lizards. Our results suggest that increasing environmental temperatures may have especially severe effects on host survival when parasites are present and highlight the need to consider interactions between thermal physiology and host–parasite dynamics when forecasting the responses of tropical animals to climate change.